This invention relates to isolators, and more particularly to isolators for isolating RF devices from one another when the cause of unwanted coupling is stray RF current flowing on the shields of coaxial lines connecting the devices together.
Unwanted coupling due to stray RF currents is very common in antenna arrays consisting of two or more single antennas stacked one above the other where each antenna must be connected to a common source. Unwanted coupling due to stray RF currents is also common in stacked or colinear antennas where each antenna must be operated in a transmit/receiver mode independently of the other antennas.
Various different systems to isolate RF devices, such as antennas, from one another when the unwanted coupling is stray RF current have been devised. An example of such a prior art system is disclosed in U.S. Pat. No. 3,879,735 issued to D. Campbell et al. In the system disclosed in this U.S. patent suppression of RF current is accomplished by using several broadband RF coaxial cable chokes on the lines connecting the antennas. The cable choke system described in the above-mentioned U.S. patent is very effective; however, the cable choke system is somewhat complicated to construct and does exhibit some frequency dispersion.
The device disclosed in U.S. Pat. No. 3,961,331 issued to D. Campbell, is an improvement of the device shown in U.S. Pat. No. 3,879,735. This improvement includes the use of damping resistors shunted across cable chokes used as isolators. These resistors broaden the attenuation characteristics of the chokes over a relatively large frequency range. By adding the shunting resistors, Campbell has decreased the quality factor or Q of the lumped resonant structure formed by the inductance and capacitance of the cable chokes. As a result, the attenuation characteristics of the cable chokes are broadened substantially over the frequency range. The U.S. Pat. No. 3,961,331 patent in FIG. 7 shows these results graphically, where in it can be seen that the addition of the shunt resistors by Campbell has substantially flattened out the attenuation characteristics over the frequency range shown.
The isolator system of this invention is an improvement of the systems disclosed in said U.S. Pat. No. 3,879,735 and U.S. Pat. No. 3,961,331 in that the isolator apparatus of this invention is relatively simple to construct, can be made more broadband, and does not exhibit frequency dispersion. Further while the invention is specifically described herein with reference to antenna systems, the isolation system of this invention can be applied wherever stray RF current suppression on shields is desired.